1. Field of the Invention
This invention relates to a method of generating micro-topography on a surface by use of a magnetic fluid. The method is useful for fabricating surface roughness master specimens, various types of holographic optical elements, master optical disks and diffraction gratings as well as for micromachining.
2. Prior Art Statement
Surface roughness master specimens, for example, are required to have fine surface irregularities. Typical of the physicochemical methods used in recent years for formation of the fine surface irregularities is one based on an optical or electron beam patterning technique employed as a processing technology in the production of semiconductor devices. This method employs means for exposing and etching various resist materials and means which takes advantage of the anisotropy of Si or the like with respect to etching for obtaining specific sectional profiles. Typical of the mechanical methods used is one employing ultra-high precision machining techniques and using means for machining a soft metal such as copper or aluminum. All of such conventional methods remove material from the surface during the formation of the fine irregularities and all have been widely adopted for fabrication of surfaces with fine irregularities.
Although laser beam patterning, electron beam patterning and other semiconductor microprocessing technologies used in the production of large-scale integrated circuits and the like are suitable for patterning two-dimensional flat surfaces, the interference arising inside the resist owing to multiple reflection of the laser beam, the backscatter of the electron beam and other problems complicate resist exposure time control and energy control and create complex etching conditions and the like. As a result, the sectional profile control is low in freedom and difficult to achieve. Moreover, patterning of a large surface area requires much time and an elaborate patterning machine with high mechanical precision and thermal stability. In ultra-high precision cutting and other mechanical machining methods, the radius of the tip of the diamond bit determines the spatial frequency of the sectional wave-form of the surface microtopography. In view of machining performance and wear resistance considerations, a practical bit is limited to a minimum tip radius of about 10 .mu.m and is thus not suitable for formation of fine surface features on the submicron order. Because of these shortcomings, the conventional methods have been unacceptable in terms of freedom of sectional profile, precision and applicability to large areas.
In addition, Japanese Patent Appln. Public Disclosure Hei 1-308918 teaches a method of fabricating a diffraction grating with fine irregularities using a magnetic fluid. This method involves using a digital signal to form a leakage magnetic field on a magnetic recording medium, coating the magnetic recording medium with a magnetic colloid, imparting the super-fine magnetic particles of the magnetic colloid with a grating pattern, covering the formed pattern with a resin and fixing the resin.
In this known method of using a digital signal for writing on the magnetic medium, however, the sectional profile of the super-fine magnetic particles formed on the magnetic medium is determined by the wavelength, as shown in FIG. 7. More specifically, as shown in FIG. 7(a), when the wavelength of the sectional profile of the irregularities (crests and valleys) formed by the deposited super-fine magnetic particles is relatively long, the profile assumes a double-humped shape at the magnetic domain walls. As shown in FIG. 7(c), when the wavelength is reduced to 26 .mu.m, the sectional profile formed by the super-fine magnetic particles assumes a wavy form with small crests and valleys and in the shortwave direction departing from the transitional region marked (T) the crests are formed at the center of the magnetic domains As shown in FIG. 7(e), when the wavelength is reduced to 15 .mu.m, the sectional profile of the super-fine magnetic particles assumes a semicircular arc form, notwithstanding that the digital signal has a square wave-form.
Thus in the conventional method of using a magnetic fluid to form irregularities, the sectional profile of the irregularities that the super-fine magnetic particles form on the magnetic medium does not change uniformly with change in wavelength. Instead, the sectional profile is determined by the wavelength, making it impossible to control the profile of the irregularities formed.
This invention was accomplished in light of the foregoing circumstances and has as its object to provide a method of generating micro-topography on a surface which enables the sectional profile formed by super-fine magnetic particles deposited on a magnetic medium to be controlled as desired.